Transverse observations of Williams patterns in the nematic liquid crystal MBBA

Abstract

Williams patterns have been studied in the nematic liquid crystal MBBA in a direction perpendicular to that in which either direct or alternating current electric fields have been applied. Both homogeneous and homeotropic films have been studied in a light microscope as a function of the orientation of polarised light and using both crossed and parallel polariser and analyser. Whereas, in essence, the Williams pattern was similar to that observed hitherto when the direction of observation was parallel to the applied field direction, the conditions of the pattern formation are different and instructive. Firstly, the setting of the polariser parallel to the prefield director appears to be the important condition for the observation of the patterns when no analyser was used. The field direction seems to have less significance. Secondly, when viewed in this transverse manner, the fluid motion, now well associated with the pattern formation, always place along the bright bands rather than perpendicular to them as in the longitudinal arrangement. This may be a function of the cell geometry. Thirdly, the equivalent focus distances are apparently longer, when transverse observation is made, than in the case of films of the same thickness viewed parallel to the field direction.